EXTERNAL CAUSES OF GROWTH AND FORMATION. II 313 



More important for us are the accessible effects of gravity as shown by ex- 

 periment. Further, in these effects also it is often that of the weight of an 

 entire organ that is concerned, a tension or pressure, and in this case gravity 

 is of course replaceable by other forces. On the other hand, gravity exerts 

 a specific influence which, even when the weight of the entire organ is eliminated 

 by being supported by a prop or by immersion in water, makes itself felt both 

 in cells and cell parts. We will begin our studies with a consideration of such 

 specific effects of gravity ; the effects of pressure and tension we may postpone 

 till later. 



The specific effect of gravity exhibits itself especially in the direction which 

 plant organs assume in space, and to which they return if they be displaced. 

 These movements are produced by changes in shape, which we will consider in 

 another connexion (Lectures XXXIV and XXXV). It may be remarked, how- 

 ever, that both in these movements and also in the effects on growth and for- 

 mation to be described here, gravity may be replaced by centrifugal force, 

 from which we may conclude that it is the ' mass -acceleration ' which 

 directly or indirectly affects the plant. The fact that gravity may be replaced 

 by centrifugal force suggests the question as to the effect of the intensity of 

 the acceleration. ELFVING'S (1880) and SCHWARZ'S (1880) experiments have 

 shown that growth is not affected by increasing or reducing the acceleration, but 

 disturbances in the arrangement of the cell-contents are induced by great centri- 

 fugal force (MoxTiER, 1899), and disturbances in growth are also to be expected. 



In nature the question of the intensity of mass-acceleration is of little conse- 

 quence, because the differences in the value of gravity on the earth's surface are 

 too small to be worth notice. The direction in which gravity acts on the plant 

 is of far more importance. Very frequently the symmetry of the plant part 

 depends on how it lies with regard to gravity, being radial when its long axis is 

 parallel with the direction of gravity, and dorsiventral when it is not so. Dorsi- 

 ventrality exhibits itself in the distribution and formation of the lateral branches ; 

 on the dorsiventral stem the branches and leaves are usually produced either 

 on the upper side only, or if they can develop all round, those on the under 

 side are distinguished by size from those on the upper (anisophylly). Roots 

 also appear to be developed only on the underside of dorsiventral organs. 

 Dorsiventrality may be recognized either at the growing point by the position 

 of the primordia of organs, or later on in the course of development of such 

 organs. The latter phenomenon is very common and is especially well seen in 

 the cuttings of the willow for example (VocHTiNG, 1878). If such a cutting 

 be suspended in its normal position in a moist chamber it forms at its apex 

 radially arranged lateral shoots, and similarly disposed roots at the base, 

 but the median region develops no lateral branches. If the cutting be placed 

 horizontally branching takes place at apex and base as before, but a number 

 of lateral shoots also appears on the upper surface from the apex backwards, and, 

 further, numerous roots arise from the under surface ; in other words the branch 

 has become dorsiventral. Finally, if the branch be suspended upside down, its 

 apex downwards and base uppermost, the largest shoots and roots appear as 

 in the first experiment, at the apex and base respectively, but both types of 

 organ develop progressively towards the opposite pole as in the normal 

 orientation. In this case a very obvious effect of gravity is disclosed, but it has 

 no power to alter the pre-existing polarity. This is very well shown in the case 

 of certain cultivated arboreal forms, e. g. the so-called ' weeping trees '. The 

 pendent branches of these plants, in spite of their inverted position, continued to 

 form lateral branches at their apices (VOCHTING, 1878). 



As yet we know of no case where gravity has induced polarity in the growing 

 point or in the ovum, as, for example, light is capable of doing in Equisetum ; 

 the possibility of such an effect, however, cannot be denied. The examples 



